In this project, we intend to investigate two major aspects of Na ion transport in the rabbit urinary bladder. (1) How is the permeability of the apical or mucosal-facing membrane regulated? That is, what factors control or regulate the passive entry of Na ion? (2) What are specific characteristics of the Na ion transport system in the basolateral or serosal facing membrane? Can we qualitatively describe the manner in which the transport system moves Na ion? To answer these questions we will examine: (1) variations of intracellular ion activities with ion specific microelectrodes; (2) conductance properties of the apical membrane after the basolateral membrane has been effectively eliminated as a permeability barrier by application of the antibiotic nystatin; and (3) the unidirectional fluxes of 22Na across the basolateral membrane after the apical membrane has been treated with nystatin. These three approaches will allow us to produce a quantitative description of Na ion transport in a mammalian epithelia tissue. Such a description has previously only been available in much simpler tissues.